Google AdSense has added the ability for publishers to target their AdSense ads to the content they think best suits their site so ads shown on their sites should only show relevant AdSense ad units.
Many sites have a lot of recurring content: headers, menu, footers and anything else you might find on every page of a site.
This can cause problems for your SERPs (diluting your keyword rich content) and the relevancy of your AdSense ads.
For example this site has a variety of content including a few posts about the World of Warcraft Gold and Leveling guides and I see a lot of ads that match this content on pages that have nothing to do with this content.


